Automatic gain control



Dec. 13, 1932. E. BRUCE 1,890,454

AUTOMATIC GAIN CONTROL Filed Dec. 21; 1,928

lPatented Dec. 1,3, 1932 UNrrED ds'mfrrzs Atiene@ PATENT OFFICE i gEDMOND BRUCE, oF am) BANK, NEW JERSEY, essrGNoR 'ro 'BELL rnLEPHONELAnOf RATORIES, INCORPORATED, or NEW YORK, N. Y., A CORPORATION on NEWYORK Y AUTOMATIC GAIN CONTROL Application filed December 21, 1928.4Serial No. 327,564.

This inventionrrelates to a gain control for -systems including spacedischarge devices and more especially to a method of and means formaintaining approximately constant the output level of the. system, eventhough the input energy varies over a wide range of values. A

Numerous suggestions have been made for maintaining at a constant levelthe output energy'ot space discharge systems supplied with input energywhich varies from time to time.

One such system, for example, is disclosed in Atel Patent 1,511,015. Inthis system the Vunmodulated component carrier wave is selected andrectified, the rectified current is passed through a resistance, andthev resultant IR drop is utilized to vary the biasing potential appliedtothe grid of the detector tube.

`Another system is disclosed in British Patent 287 ,97 2 of J une 21,1928 in which a biasing potential is obtained frointhe-IR drop through aresistance insertedl in the plate current circuit of the lowfrequencydetector.

It has been found thatthe system of the present invention aiiords anextremely sensitive control which operates to `maintain the output levelconstant to a high degree even though the input energy level vari-esbetween wide limits. .A

It is an object of this invention to maintain a constant level outputfrom a space discharge system when the input level varies between Widelimits.

A further obj ect, which is incidental to the first object, is to vary adirect current ilow between wid-e limits by a direct current whichvaries between comparatively smalllimits.

In the embodiment of the invention which space current of the detectorflows through' both `of the resistances while that of an auX- iliaryspace discharge device flows through but one of the resistances and inthe opposite direction to the space current of the detector,

thereby setting up opposing IR drops in that resistance element,f TheQpotential drop across the other resistance element, carryingv only thespace current of the detector-,"isapplied between the `gridand cathodeof the auxiliary device. The potential drop across both resistances isapplied between'thegrid and cathode of the high requencyor controldetector. When the output currentotthe low ,fre-

quency detector changes, the IR drop acrossv the tworesistances changescorrespondingly. this change in the potential drop across the resistancecarrying only the space'currentof the detector changes the biasingpotential applied to the grid ofthe auxiliary device, whereby its spacecurrent and hence the IR drop due to it changes, causing an augmentedchange in the potential which is applied to the grid of the controldetector. l T In the circuitarrangementto be described the ,spacedischarge Vdevice in which this circuit is exercised is the first orhigh frequency j detector. vHowever, it is to be understood :that theinvention is not so limited and hence the control may be effected byapplyingthe variable biasing potential to the grid of any spacedischarge device included in thes'ystem'. 1

The invention will'be better understood from the following descriptionin connejcf tion with the accompanying drawing, ,the

single figure of which shows schematically a preferred form,the'invention however-'is to be limited onlyby the scope of` theappended claims. Y In the drawing the source of energy carrying thesignals which' it is desired. tol receive is represented generallybyblcck PU. This source may bea radio antenna lsystem Or the terminalapparatus of a line wire carn riersystem etc.u Y

The incoming energy is combined wthlocally generated oscillations fromanyV convenient source 2 by means of spacedischarge device 3. The gridor control electrode of device 3 has applied to it a biasing potentialfrom source 4 of such a value that with normal strength of incomingenergy, the output of the receiver will have a desired level. Thedesired intermediate vfrequency isselected and amplified in the usualmanner Vby 1GOv . means of apparatus which is well known and lowfrequency amplifier' LFA and then further transmitted orv directlychanged into sound energy as may be desired in well known apparatusrepresented by blockL. An auxiliary space dischargeqdevice 7 isassociated with the second detector 6 in a manner to be describedhereinafter.

vSpace current for .the auxiliary device 7 Y andv for. the highfrequency detector 3 is obytainedlfrom a source. 8,'while space currentfort-he low frequency detecto-r 6 is obtained lfrom source. 9. Sources 8and 9 are repregsented `as batteries, but they may be any Vances 10 and.11. Y conditions of operation, the IR drop-through .resistances 10 and111 dueto the space current -of device 6, the opposed IR drop throughindependent sources or direct current energy.

Source 8 is'directly grounded at its negative pole while source 9 isgrounded onthe negative side through resistances 10 and 11. 'The spacecurrent'path forvlow frequency detector 6 may be'traeed from ground 12-through resistances 11 and 10, source of direct current 9, conductor 13,plate and filay ment vor" low frequency detector 6 to ground vat 14.

The space .current path for auxiliary device 7 may be traced fromgroundl, direct vcurrentsource 8, plate and filament vof de-v Yvice 47,resistance 11 to ground at `l12. It

will be noted that space current for device v7 Hows through resistance11 in an oppositedirection .to that of spacey current of device 6. v

Thevalues of resistances .10 vand 11 in one installation were 48,000ohms and 22,000 ohms respectively, with direct current sources 8'and 9of 135 volts each and gridbiasing source 4 o-221/2 volts.

The operation of .a double detection re-V ceiver is well understood andfor thatreason ythe .operation of Vthe apparatus described above will belimited to that/portion of the apparatus with which thepresent inventionis specifically concerned.

','Asmentioned above thefgrid biasing pon itential of high frequencydetector Bis such that a normal strength incoming signal will produce asignal of the desired vlevel in the.

output .of the receiver. This potential is derived from ,source 4, andthe iiow of space current for devices 6 and'7 through resist- That is,under Vnormal resistance 11 due to the space current ordevice11,'together with source 4'determines vthe grid biasing potentialforhigh frequency detector 3 while the grid bias for auxiliary device 7is determined by the space currents and VresistanceslO and 11 as will"be readily understood. y

Assume now that the incoming signal has a value larger than normal.lThiswill cause an increased viioW ofspace `current through lowAfrequency detector 6. This increased flow of current throughresistances`10 and 11 causes an increased yIR 'drop through both therebymaking the potentialof the point 16 with respect to ground 12 morenegative than' under normal'conditions. This change of po-`tentialndiderence .between the terminals of resistance 10 is effectiveon the grid of auX- .iliary device to reduce the flow of space cur-.rent therethrough and hencel the IR drop n throughresistance 11, .dueto this space current, .1s reduced.

It will be` recalled that the space fcurrentls of device flows throughlresistance 11v infan opposite direction'to the space currentof Vdevice6. The decrease in space current. of device7 therefore causes areductionin the op posed IR drop in resistance 11. This causesccfVtherpotential or ypoint :16 with yrespect 'to ground 12 to become stillmore negative. .In

vother words, the rpotential of the point 16` with respect to ground 12is made more negative due not only to the increase offplatecurrent of ythe device'6fl'o'wing through .resistances 10 and 11 but also to thedecrease in the opposed IR dropproduced by the iow of decreasedl `platecurrent of tube 7 through the resistance n NV Mit This potential ofpoint-1.6 -is added' totheA constant Lvoltage of source 4 and impressedupon the gridof thelow frequency detector v3, as' will readily appearfrom the drawing.

The increaseA of .negative potential of ,the point 16, therefore, causesthe -gridof device 3 to become more negative which in turn reduces the'overall gain of the receiver `bringing its output back tothe desiredlevel.

In case the energy level of the incoming l signal falls below thenormal' vvalue it will be'apparent that the space currents of the lowfrequency detector 6 and auxiliary dei vice 7 willichange causingthepotential drop Y across resistances 10 and 11 to fall. This de# creasein the potential drop across resistance 10 increases the plate currentof the. auxiliaryV device 7. The resulting decreasein the po-V tential.difference between the point 16 and ground 12 due lboth l,to thedecreasein space current of thedevice 6 andthe consequentincrease in thespace current of device 7 causes a decrease inthe grid biasing potentialapplied tothedevice 'and a consequentl.

increase in the overall amplification of the receiver.

No attempt has been .made yto illustrate or describe a double detectionreceiver completely as lthis .is not-believed essential. to a com-U15producing a potential in proportion to the difference in the spacecurrents of said devices, vand means for causing the space current ofthe first device to vary inversely with said potential. Y

2. In combination, a space discharge device, an auxiliary spacedischarge device, an

impedance element connected 1n the space current circuits of the twodevices so that said space `currents produce opposing potential dropstherein, means responsive to the space current of the first device forcontrolling the space current of said auxiliary device, and meansresponsive to the net potential drop in said impedance element forcontrolling the space current of the first device.

3. In combination, a signal receiving system having an output currentwith a direct current component directly proportional to the amplitudeof the signal output, means for producing an electromotive forcedirectly proportional to said direct current component, a spacedischarge device having an anode, a cathode and a control element, aspace current circuit for said device, connections fo-r supplying saidelectromotive force between said cathode and control element to controlsaid space current inversely with said electromotivel force, means forproducing a second electromotive force directly proportional to thedifference between said direct current component and said space current,and means responsive to saidsecond electromotive force for controllingthe amplitude ofthe signal output of said receiver.

4. In combination, a space discharge device, an auxiliary spacedischarge device, an impedance element so connected in the space currentcircuits of the two devices that said space currents produce opposingpotential drops therein, means for controlling the space current of saidauxiliary device inversely with the space current of the first device, asecond impedance element connected in the space current circuit of saidfirst device to the exclusion of the space current circuit of saidauxiliary device, and means responsive to the sum of the net potentialdrop in the first impedance element and the potential drop in the secondimpedance element for controlling the space current of said firstdevice.

5. In a control system, a space discharge device, a second spacedischarge device having an anode, a cathode and a control element, aresistance element connected in the anode-cathode circuits of saiddevices so that second resistance element connected in the anode-cathodecircuit of the first device to the exclusion of the anode-cathodecircuit of the second device, means for impressingbetween the cathodeand control elementof said second device a voltage ,directlyproportional to the voltage drop in said second resistance, and meansfor controlling the anode-cathode current of the first device in inverseproportion to the sum of the net voltage drop in the first resista-nceelement and the voltage drop in the second resistance element.

6. In combination, a spa-ce 4discharge d-evice, an auxiliary spacedischarge device having an anode and cathode anda control element, aresistance element connected in the space current circuit of the firstdevice, connections from said resistance element to the controlelectrode and cathode of the second device whereby the potentialdifference between said control electrode and cathode is made dependenton the space current of said first device to control the space currentvof said auxiliary device inversely with the space current of said firstdevice, al second resistance element connected in the space currentcircuits of both of said devices so that said space currents flow inopposite directions therethrough, and means for controlling the spacecurrent of said first device in response to the voltage drop Vacrosssaid second resistance element.

7. The combination according to the next preceding claim in which themeans includes a space discharge device having a cathode and a controlelement and connections for impressing the voltage drop across thesecond resistance between said cathode and control element to controlthe space current of said device inversely with said voltage drop.

In witness whereof, I hereunto subscribe my name, this 15th day ofDecember, 1928.V

EDMOND BRUCE.

